Parabolic solar troughs for generating electricity from sunlight energy, include heat collector elements. Typical heat collector elements include a pipe-like solar absorber disposed within a glass envelope, for carrying a heat transfer fluid. Because the solar absorber and the glass envelope have different coefficients of linear thermal expansion, the solar absorber and the glass envelope expand and contract longitudinally at significantly different rates. To compensate for the different linear thermal expansion and contractions, formed bellows assemblies are used for coupling the glass envelope to the solar absorber. One end of the bellows assembly is usually attached to the solar absorber by a welded joint and the other end of the bellows assembly is usually attached to the glass envelope by a glass-to-metal seal.
The solar heat collector elements heat cycle daily with temperatures approaching 400 degrees centigrade. These daily temperature fluctuations contribute to potential stresses in the glass envelope, particularly at the glass-to-metal seal. Due to the temperature range and the significantly different rates of thermal expansion between the solar absorber and the glass envelope, a significant increase in longitudinal expansion is experienced by the solar absorber with respect to the glass envelope.
The main objective of the solar absorber is to collect as much available solar radiation as possible. The solar absorber in current heat collector elements that employ formed bellows assembly at each end, have approximately 3 percent of their surface area shielded by the bellows assembly. The shielding of the solar absorber reduces the optical efficiency of the solar heat collector element.
Accordingly, a solar heat collector element with greater optical efficiency is desired.